Testing after ovulation?

Okay bare with me here....

We have just started TTC baby #2. Baby # 1 is 21 months and she took almost 2 years to concive. My period has been very odd lately, total cycle being only 21 days. I have not tracked bbt and have not really been testing for ovulation either. My PREMOM app shows I was to have Ovulated the other day. So in theory if I did an ovulation test today and had already ovulatedtwo days ago it would be SUPER faint right?

Also anyone else have short cycles? I have had two consecutive cycles that were only 21 days.